When trees don’t act their age: size-deterministic tree-ring standardization for long-tern trend estimation in shade-tolerant trees

Abstract. With increasing awareness of the consequences of climate change for global ecosystems, the focus and application of tree-ring research has shifted to reconstruction of long-term climate-related trends in tree growth. Contemporary methods for removing the biological growth-trend from tree-ring series (standardization) are ill-adapted to shade-tolerant species, leading to biases in the resultant chronology. Further, many methods, including regional curve standardization (RCS), encounter significant limitations for species in which accurate age estimation is difficult. In this study we present and test two tree-ring standardization models that integrate tree size in the year of ring formation into the estimation of the biological growth-trend. The first method, dubbed size deterministic standardization (SDS), uses tree diameter as the sole predictor of the growth-trend. The second method includes the combined (COMB) effects of age and diameter. We show that both the SDS and COMB methods reproduce long-term trends in simulated tree-ring data better than conventional methods – this result is consistent across multiple species. Further, when applied to real tree-ring data, the COMB method is more parsimonious than its than RCS. We recommend the inclusion of tree size in the year of ring formation in future tree-ring standardization models, particularly when dealing with shade-tolerant species, as it does not compromise model parsimony and allows for the inclusion of unaged trees.